One relatively new type of electrical cable now gaining increasing popularity and acceptance comprises a flexible metallic tube entirely covered with a layer of plastic or other elastomeric material. Some manufacturers of these cables fabricate the tubular metallic core from helical windings making it similar to "BX" cable, while others flute a metallic tube annularly along its length similar to the manner of forming hydroformed bellows. These configurations provide the cable with a degree of flexibility.
At least several connectors have appeared on the market which are designed specifically for connecting this type of cable to an electrical box. Each of these includes at least one compromise somewhere in its design that either unduly increases its cost, limits its use to a narrow range of cable size, or requires an unnecessarily long installation time.
An effective connector for this type of cable should preferrably be capable of assembly quickly and easily both to the cable and to an electrical box, accomodate a relatively wide range of cable sizes, provide a solid electrical grounding connection between the metal portions of the cable and the electrical box, and in some applications provide a reasonable seal between the inside of the box and the outside environment.
The connector of this invention (which will be described in greater detail later) includes a tubular connector body and a connector nut, both of which are external and visible parts, and internally it includes a retainer element, a rubber bushing and an anti-friction washer. The retainer element has the shape of a thin, narrow-rim washer with a plurality of integrally formed fingers or prongs extending radially inwardly which are formed or bent somewhat toward the electrical box end of the connector. Each of these fingers terminates short of the center of the washer so that, cumulatively, they form a central opening which is deliberately smaller than the diameter of the metallic portions of the cable. Before the cable is attached to the connector, the outer plastic coating is stripped from its end to expose these metallic portions for direct attachment of the connector thereto. Because different manufacturers produce what is supposed to be standard size cable, but which in fact has metallic portions with significantly different diameters and tolerances, these fingers in the connector assembly must be properly designed to permit a given size cable from any manufacturer to pass through the center hole of the retainer element and deflect the fingers thereon when the cable is moving therethrough in the direction of the electrical box. But, because these fingers are bent toward the electrical box to begin with, any attempt to pull the cable in the opposite direction away from the electrical box causes the fingers to dig or bite into the cable and prevent its removal. Although the retaining element is preferrably staked or swaged in place within the tubular connector body so as to always be fixed and properly oriented for use, it would also work if not affixed to the connector body, although perhaps not with a comparable grounding integrity.
The ease and simplicity of attaching the cable to the connector is further enhanced by appropriately sizing the parts such that the entire connector assembly can be shipped in assembled form with the connector nut drawn up on the connector body by hand. As will be seen, a further tightening of this connector nut (as with a wrench) causes a radial contraction of the rubber grommet to produce an inner diameter which would interfere with the free entry therethrough of the cable. Thus, since the connector nut is not drawn up tightly when shipped, the installer of the connector need only properly prepare the end of the cable (in accordance with directions supplied) and push it through the connector to lock the connector and the cable together. Then, simply by tightening down on the connector nut with an appropriate tool, the connector is further locked onto the cable and an effective sealed connection has also been made.
Because of the importance of creating a good electrical ground between the cable and the electrical box, a proper preparation of the end of the cable requires stripping a designated length of the plastic jacket from the end of the cable. Thus, the retainer element's fingers in fact bite into the metallic portions of the cable so as to electrically connect them. To ensure there exists a ground between the retainer ring and the retainer body, the retainer ring fingers, when angularly deflected by the cable, cause the annular rim portion of the retainer ring to contort and bite into the connector body. Thus, two grounding effects occur simultaneously with the mere pushing of the cable into the connector body. Electrical continuity between the connector body and the electrical box is conventionally achieved by the locknut that holds the connector in place in the electrical box.
One other effect worth mention takes place during final tightening of the connector nut. Because the grommet is compressed into the cable during final tightening, the last few turns of the nut drive the cable further inwardly such that its metallic portions rigidly compress against an internal tapered surface. This both provides additional lateral stability for the connection as well as provides supplemental ground continuity.
Thus, an installer need only strip the end of the cable, push the cable through the connector until it abuts the internal tapered surface, and tighten down the nut to complete both a solid mechanical and reliable electrical ground connection between cable and connector assembly.